Hubble Space Telescope observations of proper motions in Herbig-Haro objects 1 and 2

Hubble Space Telescope observations obtained in 1994 and 1997 are used to measure proper motions in the HH 1/2 protostellar outflow in Orion. Since the HH 1/2 system lies within 10 of the plane of the sky, proper motions provide accurate measures of true space velocities. Comparison of the 1994 and 1997 images reveals a variety of changes such as the emergence of new knots from the driving source embedded in the HH 1/2 cloud core and the fading or brightening of some features. However, such brightness changes affect a small fraction (<10%) of the total emission. Proper motion measurements reveal complex velocity variations along the flow axis and pronounced velocity shear orthogonal to it. Along the extension of the HH 1 jet axis, speeds vary from under 100 km s(-1) near the tip of HH 2, range from 255 to 345 km s(-1) in the HH 1 jet itself, and reach peak values of more than 400 km s(-1) near the leading edge of HH 1 and in parts of the brightest knot complexes in the center of HH 2. While the velocity dispersion within the low-excitation HH 1 jet is less than 30 km s(-1), high-excitation features in HH 1 and HH 2 exhibit local velocity variations in excess of 150 km s(-1). Both the internal velocity dispersion and the angular width of the emitting fluid as seen from the source, VLA 1, increase with distance. HH 1 and HH 2 contain complex substructures having chaotic internal motions, proper motions that decline rapidly orthogonal to the jet axis, and both downstream- and upstream-facing (reverse) bow shocks. Downstream-facing bow features tend to have high velocities, while reverse-facing bow shocks have low speeds. The complex texture and flow field indicate that both the fast and slow fluid elements now colliding in shocks were clumped and had chaotic velocity fields prior to entering the currently active shocks. Both fluids may have been processed by prior generations of shocks and by instabilities. Indeed, transverse motions in HH 1 and HH 2 indicate that expansion started well after the material was ejected from VLA 1, possibly as a result of having been processed through now extinct shocks. A second outflow from the HH 1/2 cloud core, HH 501, which consists of two knots located about 1" west of the base of the HH 1 jet, also has proper motions directly away from VLA 1, but with a speed of only about 180 km s(-1). The lack of interactions between the nearby high-speed HH 1 and slower HH 501 jets may indicate that the density of the medium surrounding each jet is at least an order of magnitude below that of the visible knots. Thus, the visible jet components probably transport the bulk of the energy and momentum in these outflows. Finally, the Cohen-Schwartz star is found to be a 0."2 separation binary.